Block 3 - Heat Engines:

A typical heat engine or power station takes input thermal energy, usually in the form of a fossil fuel's chemical energy, and converts this to heat energy at a lower temperature. Some of this output energy is used for kinetic and electrical energy, but a lot is wasted as excess heat energy. A combined heat and power (CHP) system would utilise a lot of the waste heat energy for space and water heating e.g. in buildings. This makes them more efficient on average than conventional power generating systems.

Diagram of energy conversion or transfer in a conventional power station

In reality there are several stages of temperature and pressure variation for the steam during the process, and generally the higher the pressure, the hotter the steam and the more kinetic energy available. However, temperature, pressure and kinetic energy become disapated at the turbine.

The 2nd Law of Thermodynamics is also the reason one cannot travel backward in time. As heat in a closed system increases, so too does entropy which also increases with time. Entropy in a closed finite system such as the universe (see Laws of Conservation for mass and energy among others) cannot be reversed by 'winding back' time. This is the Thermodynamic Time Arrow. For example, it would be virtually impossible to get back all of the original chemical energy from the burned fuel used to power a heat engine. There are current theories which hypothesize that in imaginary time it might be theoretically possible to go backward, because movement in imaginary time is synonymous with going backward in space-time (see also complex numbers).